The failures of the League in the 1930s were not only because of aggressor nations undermining its authority, but also down to its own members.
Britain and France, the two most influential members, ignored the League in their efforts to appease Hitler - actions that arguably led to the outbreak of the Second World War.
In the 1930s, the world economic depression encouraged nations to be more aggressive towards each other. Fascist dictatorships took power in Germany, Italy and Japan, which were intent on empire-building and these countries defied the League.
These crises destroyed the authority of the League, and it was powerless to stop Germany after 1935. By the time of the Sudeten crisis of 1938, Britain and France were ignoring the League, and were trying appeasement instead.